Adaptive suspension state estimation based on IMMAKF on variable vehicle speed, road roughness grade and sprung mass condition

Vehicle speed, road roughness grade and sprung mass are the three main factors to influence suspension control and state estimation. Aiming at the problem that fixed state observer cannot guarantee the estimation accuracy of suspension with driving scenario changes, a suspension state observer based on interactive multiple model adaptive Kalman filter (IMMAKF) is established. Firstly, an adaptive control suspension is proposed based on LQR algorithm and multi-objective optimization algorithm, which can automatically adjust the controller parameters according to the vehicle speed, road roughness grade and sprung acceleration parameters, so as to keep the optimal control effect of the suspension. Secondly, the theoretical model of IMMAKF is derived, and two kinds of IMMAKF suspension state observers and controllers are established. Finally, a simulation condition with the vehicle speed, road roughness grade and sprung mass changing simultaneously is established. The simulation results shows that: compared with ordinary IMMKF, AKF and KF observers, the estimation accuracy of IMMAKF5 is improved. Except for state observation, IMMAKF can be used to identify the road roughness grade and estimate the suspension sprung mass.

Suspension is a significant system for vehicles to alleviate road excitation.Active suspension systems use controlled actuator to improve ride comfort and road holding stability 1 .During the control process, the suspension controller needs to get the precise state data through state observer to calculate the optimal controller parameters.The design of controller and state observer is the core of suspension control 2,3 .
Ordinary active suspension systems mostly regard state equations as fixed values which are designed by researchers according to the most used working conditions 4 .When the vehicle is driving on special working conditions, the vibration performance like ride comfort is often poor 5,6 .Based on this problem, the adaptive suspension control strategy is proposed [7][8][9] .Adaptive suspensions can change state equations according to working conditions to optimize the vibration performance.Vehicle longitudinal speed, road roughness grade and sprung mass are the three main factors that affect the vibration performance of suspensions 10 .The calculation of these three factors has a significant impact on the control effect of the suspension.The identification of vehicle longitudinal speed can be divided into direct estimation 11,12 and indirect estimation 13,14 .At the present, with the development of sensors, the use of high-precision onboard speed sensor enables the vehicle to calculate longitudinal speed directly.The estimation of road roughness grade can be divided into two types: vision sensors based measurement [15][16][17] and state observation based measurement [18][19][20] .The second method has high accuracy, low cost and good real-time performance.For vehicles, since the total weight of passengers and cargo is random, the value of sprung mass is difficult to calculate.Therefore, the sprung mass is set as a fixed parameter by most current studies 10 or a variable parameter within a certain range.For suspensions with variable sprung mass, the sprung mass is usually set as a state parameter, and the value of sprung mass is estimated by state observation, which is complicated for modeling.Based on this problem, a new suspension controller is proposed to maintain the optimal vibration state of the suspension without calculating the exact value of the sprung mass.
The state equations of adaptive suspensions are changed with the change of working conditions (such as vehicle speed, road roughness grade and sprung mass).If state observers of adaptive suspensions are fixed, the • The IMMAKF suspension state estimation theory is proposed.IMMAKF observer can improve the state estimation accuracy under changing vehicle speed, road grade and sprung mass.• A new sprung mass estimation theory is proposed.The sprung mass is estimated by model interaction prob- abilities of IMMAKF without taking it as one of the state parameters to calculate.• A new adaptive suspension controller system is proposed.The controller system can improve the ride comfort on variable working conditions.
The rest of this paper is organized as follows.Section "Adaptive control suspension model" introduces a new adaptive suspension controller model; Section "Suspension IMMAKF State Estimation Theory" presents the theory of IMMAKF state estimation; Section "Suspension state estimation and control based on IMMAKF" details simulation and comparison and Section "Conclusions" concludes this paper.

Adaptive control suspension model
In this section, an adaptive control suspension model is established based on LQG and fuzzy control theory to make the vehicle in the optimal state when driving on different working conditions (road roughness grade, vehicle velocity and sprung mass).

LQR Half-car suspension control system modeling
In this part, the half-car suspension system is presented, and the LQR suspension control theory is discussed.
The half-car suspension model is established as Fig. 1.The suspension consists of a spring and an actuator at the front and rear part respectively.The tire is modeled as a linear spring.
Where,m H denotes the sprung mass;I y is the pitch inertia;m f and m r are front and rear unsprung mass, respectively;k f ,k r represent front and rear suspension stiffness, respectively;k tf ,k tr indicate front and rear tire stiffness, respectively;x H is the sprung mass vertical displacement at the Center of Gravity(CG) point;θ is pitch angle of the sprung mass at the CG point;x Hf and x Hr are the vertical displacement of front and rear sprung mass, respectively;x f and x r are the vertical displacement of front and rear unsprung mass, respectively;q f and Figure 1.The half-car suspension model.q r are the road input at front and rear tire, respectively; f cf and f cr are controlled actuator force at front and rear suspension, respectively; V is the longitudinal vehicle speed.
Parameters of the suspension model are shown in Table 1: Vertical dynamics equations for the half-car suspension model shown in Fig. 1 can be expressed as: where, For the suspension model, and can be approximately calculated as Equation ( 1) can be rewritten as where, According to 29 , q f and q r can be calculated as where, G q (n 0 ) is the power spectral density (PSD) of the road profile 35 .f 0 is the lowest cutoff frequency, and f 0 = 0.01Hz 36 .n 0 is the reference spatial frequency, and n 0 = 0.1m −137 .ω f and ω r denotes zero-mean band- limited white noise.The road profile is homogeneous and isotropic Gaussian process.The function of road profile 37 is shown as Eq. ( 6).
where, n k is the spatial frequency, W is the road reference coefficient, W = 2.
(1)  38 , the road roughness grades are defined from level A to D by G q (n 0 )(×10 −6 m 3 ) .The range of A grade is [0,32], B is [32,128], C is [128,512] and D is [512,2048].The road grade corresponding to the geometric mean value of G q (n 0 ) in the interval is called ISO standard road.The value of ISO A is 16, ISO B is 64, ISO C is 256 and ISO D is 1024.The ISO standard road is used to as road model in Section "Suspension state estimation and control based on IMMAKF".
According to ISO 8608 38 , the road grade index d is defined as: According to Eq. ( 7), the road grade is simplified by d.The d of ISO A is 4, ISO B is 6, ISO C is 8, ISO D is 10.According to Eq. ( 4) and ( 5), the state space equations of the half-car suspension system is expressed as Eq.(8).
Equation ( 8) is rewritten as where, To balance the ride comfort and handling stability of the vehicle under variable working conditions, the optimal controllable actuator force f cf and f cr are calculated by LQR algorithm.
The LQR controller is designed as Eq. ( 10): where J 0 is the suspension comprehensive performance index.J 0 is calculated as Eq.(11).
According to Eq. (3), J 0 is calculated as: where, q 1 and q 4 are the controller parameters of suspension deflection of front and rear suspension respectively, q 2 and q 5 are the controller parameters of sprung mass acceleration of front and rear suspension respectively, q 3 and q 6 are the controller parameters of tire deflection of front and rear tire respectively.The optimal controllable actuator force f cf and f cr are calculated by Eq. ( 13) where, P is the solution of Riccati equation 20 : When the best controllable actuator force is solved by Eq. ( 13), the optimal state space equations of LQR half-car suspension system are where, In the actual control process, according to Eq. ( 15), the system state equations is discretized as Eq. ( 16).
where, k is the sampling time, ω(k) is the process noise, and ν(k) is the sampling noise.ω(k) and ν(k) are Gaussian sequences with mean value of 0, and the time interval is 0.01 s.

Multi-objective optimization of suspension controller parameters
In this part, the controller parameters of LQR algorithm ( q 1 to q 6 ) under variable working conditions are opti- mized by the second non-dominated sorting genetic algorithm (NSGA-II).
For suspension optimal control, the optimization of the suspension controller parameters is regarded as a multi-objective optimization problem (MOOP).The road handling and ride comfort are two conflict properties.It's arduous to get a satisfactory ride comfort without sacrificing the control ability and vice versa.On the other hand, for different working conditions, the optimal LQR controller parameters are different.To keep the best suspension vibration effect under variable working conditions, LQR suspension controller parameters ( q 1 to q 6 ) are optimized by the NSGA-II algorithm.In order to facilitate the optimization process, set q 1 = q 4 , q 2 = q 5 ,q 3 = q 6 .
Since q 3 and q 6 have little influence on the driving characteristics of suspension, set q 3 = q 6 = 0.0001 .The optimization process is a MOOP.The optimization decision variables are defined as q 1 (q 4 ) and q 2 (q 5 ) .The objec- tive functions, optimization goals and constraints are shown as Eq. ( 17).
For NSGA-II optimization, the population size is 30, the number of generations is 100.The vehicle speed is 10 ~ 30 m s −1 (10, 15, 20, 25, 30 m s −1 ), road roughness grade is A ~ D class (A, B, C and D) and sprung mass is 300 ~ 500 kg (300, 350, 400, 450, 500 kg).With the combination of these conditions, 100 kinds of different conditions are obtained and optimized by NSGA-II.
The optimization results of vehicle speed 20 m s −1 , road grade C and sprung mass 400 kg are shown as Fig. 2c.The q 1 and q 2 of the point that is the closest to the origin of coordinates are selected as the best controller param- eters of this condition.The best controller parameters under various working conditions are shown in Fig. 2a,b.
According to Fig. 2a,b, with the increasing of vehicle speed V or road roughness grade index d, or the decreasing of sprung mass m H , q 1 is decreased and q 2 is increased.This is because with V and d increasing, and m H decreasing, the sprung mass acceleration |ẍ H | has an increasing tendency which deteriorates the riding comfort.Based on this, decreasing q 1 and increasing q 2 can improve the suspension deflection (i.e.x Hf − x f and |x Hr − x r | )and reduce the sprung mass acceleration (i.e.|ẍ H | ), which can improve the riding comfort.

Adaptive suspension controller modelling
In this part, according to the optimal results of q 1 and q 2 shown in Fig. 2, the fuzzy adaptive suspension controller is proposed and the parameters of the IMMAKF sub-models are calculated.
Based on the optimization results of Fig. 2, the fuzzy adaptive suspension controller model is established.Compared with the vehicle speed V and road grade d, the sprung mass m H is difficult to be directly measured.Since sprung mass acceleration ẍH is influenced by the sprung mass m H , the fuzzy controller is built with vehicle speed V, road roughness grade index d and sprung mass acceleration |ẍ H | as inputs, q 1 and q 2 as output respectively.
According to Fig. 3, with vehicle speed V, road grade index d and absolute value of vehicle acceleration |ẍ H | increase, q 1 is decreased and q 2 increased, which makes the body acceleration ẍH always be maintained at a small trend to improve the vehicle ride comfort.
The established fuzzy adaptive suspension model is simulated to calculate the parameters of IMMAKF submodels.As an example, the simulation condition is ISO C road, the vehicle speed is 20 m s −1 , the simulation duration is 250 s, and the sprung mass changes as 300-350-400-450-500 kg every 50 s.q 1 and q 2 are extracted in the simulation process, as shown in Fig. 4: According to Fig. 4, controller parameters are automatically adjusted by the fuzzy adaptive suspension model according to the working conditions.In Fig. 4, the mean value of q 1 and q 2 (as shown in the red lines in Fig. 4) are changed with sprung mass m H .When the sprung mass m H increases, the mean value of q 1 increases but q 2 S.T.0 ≤ q 1 ≤ 10 9 S.T.0 ≤ q 2 ≤ 2000 decreases.Therefore, the sprung mass m H is an important factor to affect controller parameters.The mean values of q 1 and q 2 under various working conditions are calculated as parameters of sub-models of the IMMAKF state observer, as shown in Table A.1.

Suspension IMMAKF state estimation theory
In this section, the suspension IMMAKF state estimation and control theory is provided in details.
In practice, the suspension control effect is influenced by external disturbances (like changing road roughness) and parameter uncertainties (like changing vehicle speed and sprung mass).Within these conditions, the vehicle speed can be directly measured by the on-board sensor, however the road roughness grade, sprung mass and suspension state parameters like suspension deflection and sprung mass vertical velocity are hard to be measured directly.The vibration effect of suspension is affected by the estimation precision of each state parameter.At present, Karman Filter (KF) is mostly used as state observer to calculate the real-time state.The ordinary Kalman filter has a high requirement on the accuracy of the system modelling.Since the state of adaptive suspension is changed with the change of working conditions, the ordinary Kalman filter observer is hard to satisfy the high accuracy order of state estimation of adaptive suspensions.
Since the optimal state of the vehicle under various working conditions can be preset (as shown in Table A.1), the suspension state observer of interactive multiple model adaptive Kalman filter (IMMAKF) is proposed by combining adaptive Kalman filter (AKF) and interacting multiple model Kalman filter (IMMKF) to improve the estimation accuracy of adaptive suspension.

State estimation theory of IMMAKF
In this part, the IMMAKF suspension state observer is proposed.
The IMMAKF observer is proposed based on Table A.1.Firstly, the vehicle speed V is determined by the onboard sensor, and the road grade index d is calculated according to the state observation results.Then the adaptive suspension is controlled by the fuzzy control model and the suspension state is estimated by IMMAKF observer.The control and observation process are shown in Fig. 5.
The steps of IMMAKF state estimation are shown as Step 1 to 5.
The IMMAKF state observer is formed with s IMMKF models.Each IMMKF model has r sub-models.All sub-models are Markov processes.Each IMMKF model corresponds to a grade of road, and each IMMKF submodel corresponds to a sprung mass.
Set x d,i k−1 as the original system state of sub-model i of the IMMKF model with road roughness grade index d at the step t = k-1, P d,i k−1 is the covariance matrix, and µ d,i k−1 is the probability of model i.After model interaction, the initial conditions of IMMAKF are x d,i,0 k−1 and P d,i,0 k−1 , and can be calculated as shown in Eq. ( 18) and ( 19), respectively.where, H is the system observation matrix, and R is the measurement noise covariance matrix.The Kalman filter state is calculated as: where,Z d k is the estimation value at the step t = k.The Kalman filter covariance is calculated as: Step 3: Update model probability.The observer model is updated by maximum likelihood estimation.By calculating the similarity between the current model and the current target state, the most suitable weight of the current tracking model is given at step t = k.The most matched maximum likelihood function of model i is calculated as follows: where The probability of model i is updated as: where Step 4: Combine model data. (18)

Simulation condition
In practice, the actual working conditions such as vehicle speed, road roughness grade and sprung mass can be changed simultaneously.Based on variable working conditions, referring to Table A.1, the simulation model is established as shown in Fig. 6:

IMMAKF observer models
Two kinds of IMMAKF observers are proposed by Table A.1.
Five-model IMMAKF observer (IMMAKF5): All the data in Table A.1 are selected as the sub-model of IMMAKF observer.Sub-models of IMMAKF5 cover all working conditions and the observation accuracy is high.
Three-model IMMAKF observer (IMMAKF3): the control parameters with sprung masses of 300, 400 and 500 (kg) in Table A.1 are extracted respectively as sub-models of IMMAKF3 observer.Sub-models of IMMAKF3 cover three main working conditions to maintain observation accuracy.
For comparison, data are selected from Table A.1 as other observers: ordinary Kalman filter (KF), AKF and IMMKF.Among them, the results of 20 m s −1 vehicle speed, ISO C road and 400 kg sprung mass in Table A

Road roughness grade recognition time window
Based on Eq. ( 33) to Eq. ( 37), the accuracy of road grade identification is influenced by the number of sampling points.The spatial frequency of road roughness is between [0.011, 2.83]m −1 .The minimum identification fre- quency dn is calculated as Eq. ( 42).
The minimum identification sampling step number is set as n min , then where, t is sampling interval time of adjacent sampling points, and V is vehicle speed.In this paper, t = 0.01s and V min = 10m/s,then n min ≥ 910.
The road grade roughness shown in Fig. 6 is estimated by IMMAKF5.Set n as the step number of road roughness grade recognition window.Different road grade estimation accuracy of different n is shown as Table 2.
In Table 2, the higher the number of sampling steps n, the higher the accuracy of road grade recognition, but the longer the distance for the vehicle to derive.Based on the accuracy and the driving time, the step number of road roughness grade recognition window is selected as 1250.

Comparison of simulation results
The IMMAKF5, IMMAKF3, KF, AKF and IMMKF observers are applied to the state estimation of adaptive suspension.The initial road roughness grade of IMMAKF and AKF is determined as A grade (i.e. the initial d = 4).The comparison results are shown in Fig. 7: According to Fig. 7, for all working conditions, the estimation results of IMMAKF5 is the closest to real results, followed by IMMAKF3.Since state equations of adaptive suspensions are changed with the change of vehicle speed, road roughness grade and sprung mass, the state estimation accuracy of fixed state observer is low.Therefore, in Fig. 7, the estimation accuracy of KF is the worst.Compared with KF, both AKF and IMMKF can partly follow the current suspension state (i.e.road grade and sprung mass), which makes the estimation accuracy of AKF and IMMKF lower than IMMAKF but higher than KF.
The simulation error of each observer is calculated as shown in Eq. ( 44).
where, E s is the simulation error, x ireal is the real value of x i , x iobs is the observation value of x i , n is the total number of the sampling points in the simulation, and i is the number of coefficients of state vector x.
The simulation errors of each coefficient of state vector x are shown in Table 3.
Where, IMMAKF5 is results of five-model IMMAKF observer, IMMAKF3 is results of three-model IMMAKF observer.Total is the whole simulation condition.Grade A is the simulation condition of grade A road in Fig. 6.
In Table 3, compared with other observers, the estimation error of IMMAKF3 and IMMAKF5 observers are the smallest.Within the two IMMAKF observers, IMMAKF5 has the highest accuracy.Among the four measurement vector coefficients ( ẋH θ ẋf ẋr ′ ), for the whole simulation process, compared with IMMKF, AKF and KF, the estimation error of ẋH in IMMAKF5 is reduced by 55.17%, 56.84% and 60.19% respectively; θ is reduced by 48.69%, 48.85% and 45.31% respectively; ẋf is reduced by 77.18%, 77.43% and 79.36% respectively;ẋ r is reduced by 76.78%, 77.05% and 79.43% respectively.The trend of simulation results on each grade of road is similar to the total simulation process.For other state coefficients, the accuracy of the two IMMAKF observers is also better than other observers, which verifies the superiority of the IMMAKF observer in complex working conditions.During the simulation, the road grade is calculated by IMMAKF observer.The road grade recognition results for IMMAKF5 and IMMAKF3 are shown in Fig. 8: In Fig. 8, the road roughness grade recognition results always lag behind the real road in time domain, and the lag time is just the length of a time window.Since the road grade recognition depends on sampling of q(l) , the system can't immediately calculate the road grade.During the simulation, the model interaction probability of IMMAKF3 and IMMAKF5 are shown as Fig. 9: According to Fig. 9, during the model interaction process, IMMAKF always has a sub-model with the maximum probability, which is related to the current model state, i.e. the sub-model with the maximum interaction probability in IMMAKF algorithm is the current system state model, which can be used to estimate the current sprung mass.For Fig. 9, the n steps time window is used to estimate the sprung mass of the suspension model, and the accuracy of sprung mass estimation is shown in Table 4: In Table 4, the larger the sampling step of the time window, the higher the model recognition accuracy, but the longer the model sampling time, and the longer the vehicle driving distance.When the sampling step is 500, the model has the highest accuracy and sampling time is also suitable, so 500 steps is considered as the optimal step size for sprung mass estimation.
According to Fig. 9 and Table 4, IMMAKF observers can not only ensure high estimation accuracy under variable working conditions, but also determine the sprung mass through analyzing model interaction probability.

Experiment verification
The correctness of IMMAKF state observer is verified by the suspension experiment platform.The platform is shown in Fig. 10.Since the platform is the quarter vehicle model, the suspension is used to simulate the front wheel.For the platform, the sprung mass is 2.45 kg, unsprung mass is 1 kg, the suspension stiffness is 900 N/m and the tire stiffness is 2500N/m.For the experiment, the road excitation is ISO A grade.The total experiment time is 10 s and the time interval is 0.02 s.The vehicle speed is 20 m/s.The experiment results of ordinary KF (i.e.20 m/s vehicle speed, ISO C road and 2.45 kg sprung mass) and simulation results of IMMAKF5 are selected for comparison.To show the vibration effect (i.e.ride comfort and road handling stability), the passive suspension with damping of 7.5 Ns/m is used for verification (the passive suspension doesn't have observers, the results of passive suspension are obtained by sensors and calculated as comparison).The experiment results of ẋHf are shown in Fig. 11.
The simulation results comparison is shown in Table 5.
In Fig. 11, the experiment results of IMMAKF5 is close to real values and simulation values, which verifies the accuracy of the IMMAKF5.In Table 5, the experiment errors of IMMAKF5 is smaller than KF, which verifies the superiority of IMMAKF.
In Table 5, compared with the results of KF and passive suspension, the sprung mass acceleration of IMMAKF5 is improved by 14.64% and 62.73% respectively; the suspension deflection is improved by 11.43%

Conclusions
In this paper, the IMMAKF suspension state observer is proposed for the state observation of the adaptive suspension model under variable working conditions (vehicle speed, road roughness grade and sprung mass).Firstly, the adaptive suspension controller is established.The LQR controller parameters under variable working conditions are optimized by NSGA-II algorithm.Referring to the optimal results, the fuzzy adaptive suspension controller is presented.The adaptive suspension can automatically change the optimal controller parameters by working conditions.Furthermore, the IMMAKF suspension state estimation theory is discussed in detail.Based on IMMKF and AKF, an IMMAKF suspension state observer is established, and the theoretical equations of the algorithm are derived.Lastly, the simulation and experiment results show that, compared with state observers of KF, AKF and IMMKF, the accuracy of the IMMAKF state observer is the highest and the ride comfort of IMMAKF is improved.Except for high estimation accuracy, the road roughness grade and sprung mass can be calculated by IMMAKF observer.Therefore, IMMAKF is effective for high suspension estimation accuracy and the adaptive control on complex conditions.Note that the proposed method is a suspension state observer, which is established based on the typical working conditions, so the application of this proposed method is limited.The state estimation schema of full car suspension model under more complex working conditions such as the longitudinal vehicle speed with acceleration and the vehicle driving on slope roads, will be investigated in our future study.In addition, the road roughness grade identification by visual sensors will also be considered in the future study. https://doi.org/10.1038/s41598-023-49766-ywww.nature.com/scientificreports/ min |ẍ H |(RMS) min 0.5(max( x Hf − x f ) + max(|x Hr − x r |)) S.T. max( x Hf − x f ) < 0.15 S.T. max(|x Hr − x r |) < 0.15

Figure 3 .
Figure 3.The fuzzy control relationships.(a, b, c) the relationship between d, V,|ẍ H | and q 1 ; (d, e, f) the relationship between d, V,|ẍ H | and q 2 .

Figure 5 .
Figure 5. Flow chart of observation and control of IMMAKF.

Figure 7 .
Figure 7. Model simulation and comparison results: (a) results of ẋH (b) results of θ (c) results of x H (d) results of θ.

Table 1 .
Parameters of the half-car suspension model.
is the mix probability of model j transferred to model i of the IMMKF model with road grade index d at the step t = k-1, and p d ji is the transition probability matrix from model j to model i.

Table 2 .
Road grade estimation accuracy.

Table 3 .
Comparison of simulation error.

Table 4 .
Sprung mass estimation accuracy of IMMAKF observers.

type Parameters Model IMMAKF5 (Simulation) IMMAKF5 (Experiment) KF Passive
38% respectively, which verifies that the IMMAKF observer can improve the ride comfort and balance the handling stability of the vehicle.